Typically, fiber-optic based telecommunications networks employ optical switches to switch signals arriving along a number of optical fibers. FIG. 1 shows an example of an optical switch 100, adapted to receive a plurality of wavelength division multiplexed (WDM) input signals SIN,A . . . SIN,N on a respective plurality of input optical fibers 110A . . . 110N and adapted to output a corresponding set of WDM output signals SOUT,A . . . SOUT,N on a respective plurality of output optical fibers 120A . . . 120N. Each of the WDM input signals and WDM output signals may carry up to M single-carrier optical signals. Thus, viewed from outside the switch 100, there is provided switching for M×N single-carrier optical signals.
Inside the optical switch 100 there is provided a plurality of wavelength division demultiplexing (WDD) devices 130A . . . 130N for splitting up respective ones of the WDM input signals SIN,A . . . SIN,N into their single-carrier components. At the output end of the optical switch 100, there is provided a plurality of wavelength division multiplexing (WDM) devices 140A . . . 140N for recombining respective sets of single-carrier optical signals into the WDM output signals SOUT,A . . . SOUT,N, respectively.
Disposed between the WDD devices 130A . . . 130N and the WDM devices 140A . . . 140N is a switching core 150, which provides switching functionality for the array of single-carrier optical signals arriving from the WDD devices 130A . . . 130N. The switching core 150 is responsive to a connection map provided by a switch controller 160 which interfaces with network components that are external to the optical switch 100.
The switching activity performed by the switching core 150 may occur in the electrical domain (with the aid of opto-electronic converters) or in the optical domain or in a combination of the two. As the number of single-carrier optical signals required to be switched by the optical switch 100 grows beyond several hundred, it becomes increasingly efficient to perform at least part of the switching core's functionality purely in the optical domain. Such optical switches are known as photonic switches.
An example of a photonic switch is shown in FIG. 2 and is described more fully in the U.S. patent application Ser. No. 09/511,065 to Graves et al., entitled “Switch for Optical Signals”, filed on Feb. 23, 2000, assigned to the assignee of the present invention and hereby incorporated by reference herein. The switching core 250 of the photonic switch 200 provides controllable switching of the single-carrier optical signals received from the WDD devices 130A . . . 130N. In the illustrated embodiment, the switching core 250 comprises M optical switch matrices 210A . . . 210M, one for each of the M optical wavelengths in the system, as well as a wavelength converting switch 220.
Each of the optical switch matrices 210A . . . 210M has a fixed number of input ports and output ports and can (but need not) be a Micro-Electro-Mechanical System (MEMS) device similar in concept to, or as described in “Free-Space Micromachined Optical-Switching Technologies and Architectures” by Lih Y. Lin of AT&T Labs-Research during OFC99 Session W14-1 on Feb. 24, 1999. This article is incorporated by reference herein.
As described in the above-referenced article, a MEMS device comprises a set of mirrors that are arranged in geometrical relationship with the input and output ports in such a way that incoming light from any input port can be diverted to any output port by raising an appropriate one of the mirrors. The raising and lowering of mirrors is performed under control of the switch controller 160.
In fact, each of the optical switch matrices 210A . . . 210M has a total of K+N input ports and K+N output ports where N is the number of WDM input signals and WDM output signals. For each of the optical switch matrices 210A . . . 210M, each of the N input ports is connected to the like-wavelength output port of a respective one of the WDD devices 130A . . . 130N, while the remaining K input ports are connected to the wavelength converting switch 220.
The signals exiting the optical switch matrices 210A . . . 210M through the output ports thereof can be referred to as “switched” single-carrier optical signals and are shown at 260 in FIG. 2. Among the N+K switched single-carrier optical signals exiting a particular one of the switching matrices 210A . . . 210M, N of these are fed to like-wavelength input ports of the WDM devices 140A . . . 140N, while the remaining K signals are fed to the wavelength converting switch 220.
The wavelength converting switch 220 thus receives and outputs M×K single-carrier optical signals. In order to provide the required wavelength conversion functionality, the wavelength converting switch 220 is equipped with circuitry for converting the received single-carrier optical signals into electronic form, electrically switching the electronic signals and then modulating each switched electronic signal in accordance with an optical source at a desired wavelength. Wavelength conversion is particularly useful when an input wavelength is already in use along the fiber path leading to a destination WDM device. It is also useful as a regeneration function when the optical signal has accumulated too many impairments for onward direct optical propagation.
The arrangement of FIG. 2 permits optical signals of a given wavelength entering any particular optical switch matrix (associated with a particular wavelength) to be connected in a non-blocking fashion to any like-wavelength input port on any of the WDM devices 140A . . . 140N. Moreover, provided sufficient switching capacity is provided in the wavelength converting switch 220, the wavelength of a number of single-carrier optical signals can be changed so that each of these signals may appear on any input port of any of the WDM devices 140A . . . 140N.
Since wavelength conversion is a relatively expensive process, a trade-off exists between the level of wavelength flexibility available at the switch 200 and the cost of the switch. Fortunately, with network-level control of the wavelengths used by the various optical sources in the network, it is usually possible to ensure that most wavelengths can transit directly across most nodes in the network without requiring wavelength conversion. This is particularly true in a network having a mesh topology, which is becoming the favoured topology for new networks. Thus, even for a relatively small value for K, it is usually possible to achieve a minimal blocking probability at the switch 200.
It is also noted that the wavelength converting switch 220 may accept a plurality of add carriers on a plurality (R) of “add” paths 270 and similarly may output a plurality of drop carriers on a plurality (R) of “drop” paths 280. For simplicity, the term “wavelength converting switch” will be used throughout the following, with the understanding that a “wavelength converting switch” may have either wavelength conversion capabilities or add/drop capabilities or both.
The switch controller 160 generates a connection map under external or locally generated stimulus and provides this connection map to the components of the switching core 250. Specifically, each optical switch matrix executes a respective (N+K)×(N+K) mapping in the optical domain and the wavelength converting switch 220 executes an (M×K)×(M×K) mapping in the electrical domain.
The photonic switch described in part herein above and described in more detail in U.S. patent application Ser. No. 09/511,065 is an example of how developments in the field of optical switching are often stimulated by the need to accommodate the ever increasing optical wavelength density of WDM networks in general and WDM signals in particular.
Another consequence of the increasing density of emerging high-capacity WDM systems is an increased probability with which the switching of single-carrier optical signals within the optical switch 100 can be made erroneously or, in some cases, not made at all. Some of the myriad causes of mis-connections and lost connections include stuck or failed switch elements in the switching core 150, hardware or software failures causing incorrect switch path instructions to be received by the switching core 150 from the switch controller 160, human error (e.g., a mis-connected fiber interconnect into or between bays of switching equipment), etc.
Given the high line rates currently used in WDM networks and the even higher line rates contemplated for use in the foreseeable future, it is clear that erroneous or lost connections can and will have a very severe negative impact on quality of service by causing the loss of large amounts of data. It is therefore of prime importance to verify the integrity of connections established by the switching core 150 in order to ensure that these indeed correspond to the connections specified by the connection map stored in the switch controller 160.